8.3. read系统调用的实现

在文件~/repos/build_all/fp_octeon_56xx/build/linux/fs/rea
d_write.c中实现了read系统调用。
首先它调用宏SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)来定义一个带三个参数的系统调用。

让我们来看看这个宏:
~/repos/build_all/fp_octeon_56xx/build/linux/includ
e/linux/syscalls.h中定义了它。
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINEx(x, sname, ...)				\
	__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

#define __SYSCALL_DEFINEx(x, name, ...)					\
	asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__));  
 d_write.c
#define __SC_DECL1(t1, a1)	t1 a1
#define __SC_DECL2(t2, a2, ...) t2 a2, __SC_DECL1(__VA_ARGS__)
#define __SC_DECL3(t3, a3, ...) t3 a3, __SC_DECL2(__VA_ARGS__)

最终SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)经过预处理后变成了
asmlinkage long sys_read (unsigned int fd, char __user *buf, size_t count);
也就是说这个函数原型声明在syscalls.h,但是定义和实现在read_write.c

上面已经分析过,系统调用中断处理是从系统调用表中查找到真正入口函数跳转过去的,sys_read自然也会在这个表中。
"~/repos/build_all/fp_octeon_56xx/build/linux/arch/mips/kernel/scall64-64.S
可以找到这个表:
	.align	3
	.type	sys_call_table,@object
EXPORT(sys_call_table)
	PTR	sys_read			/* 5000 */
	PTR	sys_write
	PTR	sys_open
	PTR	sys_close
	PTR	sys_newstat
	PTR	sys_newfstat			/* 5005 */
	PTR	sys_newlstat
	PTR	sys_poll
	PTR	sys_lseek
	PTR	sys_mips_mmap
	PTR	sys_mprotect			/* 5010 */
	PTR	sys_munmap
	PTR	sys_brk
	PTR	sys_rt_sigaction
	PTR	sys_rt_sigprocmask
	PTR	sys_ioctl			/* 5015 */
	PTR	sys_pread64
	PTR	sys_pwrite64
	PTR	sys_readv
	PTR	sys_writev
	PTR	sys_access			/* 5020 */
	PTR	sysm_pipe
	PTR	sys_select
	PTR	sys_sched_yield
	PTR	sys_mremap
	PTR	sys_msync			/* 5025 */
	PTR	sys_mincore
	PTR	sys_madvise
	PTR	sys_shmget
	PTR	sys_shmat
	PTR	sys_shmctl			/* 5030 */
	PTR	sys_dup
	PTR	sys_dup2
	PTR	sys_pause
	PTR	sys_nanosleep
	PTR	sys_getitimer			/* 5035 */
	PTR	sys_setitimer
	PTR	sys_alarm
	PTR	sys_getpid
	PTR	sys_sendfile64
	PTR	sys_socket			/* 5040 */
	PTR	sys_connect
	PTR	sys_accept
	PTR	sys_sendto
	PTR	sys_recvfrom
	PTR	sys_sendmsg			/* 5045 */
	PTR	sys_recvmsg
	PTR	sys_shutdown
	PTR	sys_bind
	PTR	sys_listen
	PTR	sys_getsockname			/* 5050 */
	PTR	sys_getpeername
	PTR	sys_socketpair
	PTR	sys_setsockopt
	PTR	sys_getsockopt
	PTR	sys_clone			/* 5055 */
	PTR	sys_fork
	PTR	sys_execve
	PTR	sys_exit
	PTR	sys_wait4
	PTR	sys_kill			/* 5060 */
	PTR	sys_newuname
	PTR	sys_semget
	PTR	sys_semop
	PTR	sys_semctl
	PTR	sys_shmdt			/* 5065 */
	PTR	sys_msgget
	PTR	sys_msgsnd
	PTR	sys_msgrcv
	PTR	sys_msgctl
	PTR	sys_fcntl			/* 5070 */
	PTR	sys_flock
	PTR	sys_fsync
	PTR	sys_fdatasync
	PTR	sys_truncate
	PTR	sys_ftruncate			/* 5075 */
	PTR	sys_getdents
	PTR	sys_getcwd
	PTR	sys_chdir
	PTR	sys_fchdir
	PTR	sys_rename			/* 5080 */
	PTR	sys_mkdir
	PTR	sys_rmdir
	PTR	sys_creat
	PTR	sys_link
	PTR	sys_unlink			/* 5085 */
	PTR	sys_symlink
	PTR	sys_readlink
	PTR	sys_chmod
	PTR	sys_fchmod
	PTR	sys_chown			/* 5090 */
	PTR	sys_fchown
	PTR	sys_lchown
	PTR	sys_umask
	PTR	sys_gettimeofday
	PTR	sys_getrlimit			/* 5095 */
	PTR	sys_getrusage
	PTR	sys_sysinfo
	PTR	sys_times
	PTR	sys_ptrace
	PTR	sys_getuid			/* 5100 */
	PTR	sys_syslog
	PTR	sys_getgid
	PTR	sys_setuid
	PTR	sys_setgid
	PTR	sys_geteuid			/* 5105 */
	PTR	sys_getegid
	PTR	sys_setpgid
	PTR	sys_getppid
	PTR	sys_getpgrp
	PTR	sys_setsid			/* 5110 */
	PTR	sys_setreuid
	PTR	sys_setregid
	PTR	sys_getgroups
	PTR	sys_setgroups
	PTR	sys_setresuid			/* 5115 */
	PTR	sys_getresuid
	PTR	sys_setresgid
	PTR	sys_getresgid
	PTR	sys_getpgid
	PTR	sys_setfsuid			/* 5120 */
	PTR	sys_setfsgid
	PTR	sys_getsid
	PTR	sys_capget
	PTR	sys_capset
	PTR	sys_rt_sigpending		/* 5125 */
	PTR	sys_rt_sigtimedwait
	PTR	sys_rt_sigqueueinfo
	PTR	sys_rt_sigsuspend
	PTR	sys_sigaltstack
	PTR	sys_utime			/* 5130 */
	PTR	sys_mknod
	PTR	sys_personality
	PTR	sys_ustat
	PTR	sys_statfs
	PTR	sys_fstatfs			/* 5135 */
	PTR	sys_sysfs
	PTR	sys_getpriority
	PTR	sys_setpriority
	PTR	sys_sched_setparam
	PTR	sys_sched_getparam		/* 5140 */
	PTR	sys_sched_setscheduler
	PTR	sys_sched_getscheduler
	PTR	sys_sched_get_priority_max
	PTR	sys_sched_get_priority_min
	PTR	sys_sched_rr_get_interval	/* 5145 */
	PTR	sys_mlock
	PTR	sys_munlock
	PTR	sys_mlockall
	PTR	sys_munlockall
	PTR	sys_vhangup			/* 5150 */
	PTR	sys_pivot_root
	PTR	sys_sysctl
	PTR	sys_prctl
	PTR	sys_adjtimex
	PTR	sys_setrlimit			/* 5155 */
	PTR	sys_chroot
	PTR	sys_sync
	PTR	sys_acct
	PTR	sys_settimeofday
	PTR	sys_mount			/* 5160 */
	PTR	sys_umount
	PTR	sys_swapon
	PTR	sys_swapoff
	PTR	sys_reboot
	PTR	sys_sethostname			/* 5165 */
	PTR	sys_setdomainname
	PTR	sys_ni_syscall			/* was create_module */
	PTR	sys_init_module
	PTR	sys_delete_module
	PTR	sys_ni_syscall			/* 5170, was get_kernel_syms */
	PTR	sys_ni_syscall			/* was query_module */
	PTR	sys_quotactl
	PTR	sys_nfsservctl
	PTR	sys_ni_syscall			/* res. for getpmsg */
	PTR	sys_ni_syscall			/* 5175  for putpmsg */
	PTR	sys_ni_syscall			/* res. for afs_syscall */
	PTR	sys_ni_syscall			/* res. for security */
	PTR	sys_gettid
	PTR	sys_readahead
	PTR	sys_setxattr			/* 5180 */
	PTR	sys_lsetxattr
	PTR	sys_fsetxattr
	PTR	sys_getxattr
	PTR	sys_lgetxattr
	PTR	sys_fgetxattr			/* 5185 */
	PTR	sys_listxattr
	PTR	sys_llistxattr
	PTR	sys_flistxattr
	PTR	sys_removexattr
	PTR	sys_lremovexattr		/* 5190 */
	PTR	sys_fremovexattr
	PTR	sys_tkill
	PTR	sys_ni_syscall
	PTR	sys_futex
	PTR	sys_sched_setaffinity		/* 5195 */
	PTR	sys_sched_getaffinity
	PTR	sys_cacheflush
	PTR	sys_cachectl
	PTR	sys_sysmips
	PTR	sys_io_setup			/* 5200 */
	PTR	sys_io_destroy
	PTR	sys_io_getevents
	PTR	sys_io_submit
	PTR	sys_io_cancel
	PTR	sys_exit_group			/* 5205 */
	PTR	sys_lookup_dcookie
	PTR	sys_epoll_create
	PTR	sys_epoll_ctl
	PTR	sys_epoll_wait
	PTR	sys_remap_file_pages		/* 5210 */
	PTR	sys_rt_sigreturn
	PTR	sys_set_tid_address
	PTR	sys_restart_syscall
	PTR	sys_semtimedop
	PTR	sys_fadvise64_64		/* 5215 */
	PTR	sys_timer_create
	PTR	sys_timer_settime
	PTR	sys_timer_gettime
	PTR	sys_timer_getoverrun
	PTR	sys_timer_delete		/* 5220 */
	PTR	sys_clock_settime
	PTR	sys_clock_gettime
	PTR	sys_clock_getres
	PTR	sys_clock_nanosleep
	PTR	sys_tgkill			/* 5225 */
	PTR	sys_utimes
	PTR	sys_mbind
	PTR	sys_ni_syscall			/* sys_get_mempolicy */
	PTR	sys_ni_syscall			/* sys_set_mempolicy */
	PTR	sys_mq_open			/* 5230 */
	PTR	sys_mq_unlink
	PTR	sys_mq_timedsend
	PTR	sys_mq_timedreceive
	PTR	sys_mq_notify
	PTR	sys_mq_getsetattr		/* 5235 */
	PTR	sys_ni_syscall			/* sys_vserver */
	PTR	sys_waitid
	PTR	sys_ni_syscall			/* available, was setaltroot */
	PTR	sys_add_key
	PTR	sys_request_key			/* 5240 */
	PTR	sys_keyctl
	PTR	sys_set_thread_area
	PTR	sys_inotify_init
	PTR	sys_inotify_add_watch
	PTR	sys_inotify_rm_watch		/* 5245 */
	PTR	sys_migrate_pages
	PTR	sys_openat
	PTR	sys_mkdirat
	PTR	sys_mknodat
	PTR	sys_fchownat			/* 5250 */
	PTR	sys_futimesat
	PTR	sys_newfstatat
	PTR	sys_unlinkat
	PTR	sys_renameat
	PTR	sys_linkat			/* 5255 */
	PTR	sys_symlinkat
	PTR	sys_readlinkat
	PTR	sys_fchmodat
	PTR	sys_faccessat
	PTR	sys_pselect6			/* 5260 */
	PTR	sys_ppoll
	PTR	sys_unshare
	PTR	sys_splice
	PTR	sys_sync_file_range
	PTR	sys_tee				/* 5265 */
	PTR	sys_vmsplice
	PTR	sys_move_pages
	PTR	sys_set_robust_list
	PTR	sys_get_robust_list
	PTR	sys_kexec_load			/* 5270 */
	PTR	sys_getcpu
	PTR	sys_epoll_pwait
	PTR	sys_ioprio_set
	PTR	sys_ioprio_get
	PTR	sys_utimensat			/* 5275 */
	PTR	sys_signalfd
	PTR	sys_ni_syscall
	PTR	sys_eventfd
	PTR	sys_fallocate
	PTR	sys_timerfd_create		/* 5280 */
	PTR	sys_timerfd_gettime
	PTR	sys_timerfd_settime
	PTR	sys_signalfd4
	PTR	sys_eventfd2
	PTR	sys_epoll_create1		/* 5285 */
	PTR	sys_dup3
	PTR	sys_pipe2
	PTR	sys_inotify_init1
	PTR	sys_preadv
	PTR	sys_pwritev			/* 5390 */
	PTR	sys_rt_tgsigqueueinfo
	PTR	sys_perf_event_open
	PTR	sys_accept4
	PTR     sys_recvmmsg
	.size	sys_call_table,.-sys_call_table

最后要提供给应用程序使用read这个系统调用,所以在unistd.h里提供了系统调用号和系统调用的原型。
#define __NR_read 64
__SYSCALL(__NR_read, sys_read)